Digital engine control apparatus and method

ABSTRACT

The disclosure describes digital apparatus and a digital method for operating a multicylinder internal combustion engine having spark plugs that are fired in response to an electrical fire signal and having fuel injectors that are opened to admit a quantity of fuel in response to an electrical fuel signal having a duration determined by the speed of the engine and the position of a throttle controlled by an operator of the engine.

United States Patent Anderson, Jr.

DIGITAL ENGINE CONTROL APPARATUS AND METHOD Robert L. Anderson, Jr., Pittsburgh. Pa.

Essex International, Inc., Fort Wayne, 1nd.

Filed: Dec, 29, 1972 Appl, No.: 319,345

lnventor:

Assignee:

US. Cl.. 123/32 EA, 123/139 AW, 123/119 R, 123/117 R, 23S/l50.21, 340/1725 Int. Cl. F02m 52/00 Field of Search..... 123/32 EA, 119 R, 139 AL, 123/139 R, 139 AW, [40 MC, 140 CC; 235/150, 1502, 150,21; 340/1725 References Cited UNITED STATES PATENTS 1/1959 Reggio 123/140 CC 9/1962 Schmidt 123/139 AL Sept. 17, 1974 3,606,869 9/l97l Hunlzingcr ct a1 H 123/139 AW 3,659,563 5/1972 Monpetit H 123/32 EA 3,689,753 9/1972 Williams et al, 235/15021 FOREIGN PATENTS OR APPLICATIONS 2,004,269 8/1970 Germany 123/32 EA Primary ExaminerLaurence M. Goodridge Attorney, Agent, or FirmMolinare, Allegretti, Newitt & Witcoff [57] ABSTRACT The disclosure describes digital apparatus and a digital method for operating a multicylinder internal combustion engine having spark plugs that are fired in response to an electrical fire signal and having fuel injectors that are opened to admit a quantity of fuel in response to an electrical fuel signal having a duration determined by the speed of the engine and the position of a throttle controlled by an operator of the engme.

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1. In a fuel system for an internal combustion engine including a source of fuel for a fuel injection means that is opened to admit a predetermined quantity of fuel to the engine in response to an electrical fuel signal, improved apparatus for controlling the fuel signal comprising: first means for generating a first engine condition digital number having a value representing a first operating characteristic of the engine; second means for generating a second engine condition digital number having a value representing a second operating characteristic of the engine; data memory means for storing each of a plurality of fuel digital words representing a plurality of discrete engine fuel requirements; program memory means for storing a plurality of instruction digital words representing a plurality of operating instructions; a program sequencer for initiating the operating instructions in a predetermined order; selecting means responsive to the operating instructions for producing a resultant fuel digital word representing the engine fuel requirement by selecting one or more of the fuel digital words depending on the values of the first and second engine condition digital numbers; means for storing the resultant fuel digital word; means for generating a position signal when the engine is in a predetermined position; means responsive to the position signal for producing said fuel signal at the end of a predetermined operating interval; and means for altering the value of the fuel signal as a function of the value of the resultant fuel digital word so that the proper quantity of fuel is admitted to the engine through the fuel injector means.
 2. Apparatus, as claimed in claim 1, wherein the first and second means each comprises an analog to digital converter.
 3. Apparatus, as claimed in claim 1, wherein the data memory means comprises means for storing each fuel digital word at a predetermined address that represents a predetermined value of the first engine operating characteristic and a predetermined value of the second engine operating characteristic, and wherein the selecting means comprises: means responsive to the first and second engine condition digital numbers for selecting a plurality of the predetermined addresses, each selected address representing values of the first and second engine operating characteristics that at least approximately correspond to the values of the first and second engine operating characteristics represented by the first and second engine condition digital numbers; and means for deriving from the fuel digital words stored at the selected addresses a single resultant fuel digital word.
 4. Apparatus, as claimed in claim 3, wherein the data memory means comprises means for uniquely identifying each predetermined address by a preselected digital first address number proportional to a preselected value of the first operating characteristic and by a preselected digital second address number proportional to a preselected value of the second operating characteristic.
 5. Apparatus, as claimed in claim 4, wherein the selecting means comprises: means for comparing the first address numbers with the first engine condition digital number and for comparing the second address numbers with the second engine condition digital number, and for selecting a plurality of selected addresses identified by first and second address numbers thAt approximate the values of the first and second engine condition digital numbers; and means for interpolating between the fuel digital words stored at the selected addresses to produce the resultant fuel digital word.
 6. Apparatus, as claimed in claim 1, wherein the selecting means comprises: an arithmetic unit for interpolating between the selected fuel digital words in accordance with the operating instructions.
 7. Apparatus, as claimed in claim 1, wherein the means for generating a position signal comprises: a first member mounted to a fixed portion of the engine; a second member mounted to a cyclically moving portion of the engine so that the second member periodically passes adjacent the first member; and means for producing the position signal when the second member passes adjacent the first member.
 8. Apparatus, as claimed in claim 1, wherein the means responsive to the position signal comprises: a clock generator for generating clock pulses at a predetermined rate; a counter; means for storing an open-time digital number having a value proportional to said predetermined operating interval; means for loading the open-time digital number into the counter in response to the position signal; means for transmitting clock pulses to the counter so that the counter counts to a predetermined value; and means responsive to the counter for commencing the production of the fuel signal after the counter has counted to the predetermined value.
 9. Apparatus, as claimed in claim 1, wherein the means for altering comprises: a clock generator for generating clock pulses at a predetermined rate; a counter; means for loading the resultant fuel digital word into the counter in response to the commencement of the fuel signal; means for transmitting the clock pulses to the counter so that the counter counts to a predetermined value; and means for terminating the fuel signal when the counter has counted to the predetermined value.
 10. Apparatus, as claimed in claim 1, wherein the internal combustion engine comprises a first cylinder and a second cylinder, wherein the fuel injection means comprises a first fuel injector for admitting fuel to the first cylinder and a second fuel injector for admitting fuel to the second cylinder, wherein the fuel signal comprises a first current pulse for controlling the first fuel injector and a second current pulse for controlling the second fuel injector, wherein the means responsive to the position signal comprises means for commencing the production of the first current pulse at the end of a first predetermined operating interval and means for commencing the production of the second current pulse at the end of a second predetermined operating interval, and wherein the means for altering comprises means for separately altering the values of the first and second current pulses so that a quantity of fuel proportional to the value of the resultant fuel digital word is separately admitted to the first cylinder through the first fuel injector and to the second cylinder through the second fuel injector.
 11. Apparatus, as claimed in claim 10, wherein the means for storing comprises means for storing a first correction digital number and a second correction digital number and wherein the means for altering comprises means for altering the value of the first current pulse so that a quantity of fuel is admitted to the first cylinder proportional to the combined values of the resultant fuel digital word and the first correction digital number and means for altering the value of the second current pulse so that a quantity of fuel is admitted to the second cylinder proportional to the combined values of the resultant fuel digital word and the second correction digital number.
 12. In a control system for an internal combustion engine including means for operating a controlled member in response to an electrical control signal, improved apparatus for controlling the control signal comprising: first means for generating a first engine condition digital number having a value representing a first operating characteristic of the engine; second means for generating a second engine condition digital number having a value representing a second operating characteristic of the engine; data memory means for storing each of a plurality of control digital words representing a plurality of control requirements; program memory means for storing a plurality of instruction digital words representing a plurality of operating instructions; a program sequencer for initiating the operating instructions in a predetermined order; selecting means responsive to the operating instructions for producing a resultant control digital word representing the engine control requirement by selecting one or more of the control digital words depending on the values of the first and second engine condition digital numbers; means for storing the resultant control digital word; and means for generating a position signal for controlling the control signal as a function of the value of the resultant control digital word.
 13. Apparatus, as claimed in claim 12, wherein the control system comprises an ignition system, wherein the means for operating comprises a means for producing a spark in order to explode a fuel mixture inside the engine, wherein the control signal comprises an electrical fire signal; and wherein the resultant control digital word represents a spark timing requirement of the engine.
 14. Apparatus, as claimed in claim 12, wherein the data memory means comprises means for storing each control digital word at a predetermined address that represents a predetermined value of the first engine operating characteristic and a predetermined value of the second engine operating characteristic, and wherein the selecting means comprises: means responsive to the first and second engine condition digital numbers for selecting a plurality of the predetermined addresses, each selected address representing values of the first and second engine operating characteristics that at least approximately correspond to the values of the first and second engine operating characteristics represented by the first and second engine condition digital numbers; and means for deriving from the control digital words stored at the selected addresses a single resultant control digital word.
 15. Apparatus, as claimed in claim 14, wherein the data memory means comprises means for uniquely identifying each predetermined address by a preselected digital first address number proportional to a preselected value of the first operating characteristic and by a preselected digital second address number proportional to a preselected value of the second operating characteristic.
 16. Apparatus, as claimed in claim 15, wherein the selecting means comprises means for comparing the first address numbers with the first engine condition digital number and for comparing the second address numbers with the second engine condition digital number, and for selecting a plurality of the predetermined addresses identified by first and second address numbers that approximate the values of the first and second engine condition digital numbers; and means for interpolating between the control digital words stored at the selected addresses to produce the resultant control digital word.
 17. Apparatus, as claimed in claim 12, wherein the selecting means comprises: an arithmetic unit for interpolating between the selected control digital words in accordance with the operating instructions.
 18. Apparatus, as claimed in claim 12, wherein the means for generating a position signal comprises: a first member mounted to a fixed portion of the engine; a second member mounted to a cyclically moving portion of the engine so that the second member periodically passes adjacent the first member; and means for producing the position signal when the second member passes adjacent the first member.
 19. Apparatus, as claimed in claim 12, wherein the means responsive to the position signal comprises: a clock generator for generating clock pulses at a predetermined rate; a counter; means for loading the resultant control digital word into the counter in response to the position signal; means for transmitting the clock pulses to the counter so that the counter counts to a predetermined value; and means responsive to the counter for producing the control signal after the counter has counted to the predetermined value.
 20. In a system for controlling the operation of an internal combustion engine including a source of fuel for a fuel injection means that is opened to admit a predetermined quantity of fuel to the engine in response to an electrical fuel signal and a spark means for producing a spark cable of igniting the fuel inside the engine in response to a fire signal, improved apparatus for controlling the fuel signal and the timing of the fire signal comprising: first means for generating a first engine condition digital number having a value representing a first operating characteristic of the engine; second means for generating a second engine condition digital number having a value representing a second operating characteristic of the engine; data memory means for storing each of a plurality of fuel digital words representing a plurality of discrete engine fuel requirements and for storing each of a plurality of ignition digital words representing a plurality of ignition timing requirements for the engine; selecting means for producing a resultant fuel digital word for controlling the amount of fuel injected into the engine and a resultant ignition digital word for controlling the ignition timing of the engine by selecting one or more of the fuel digital words and one or more of the ignition digital words depending on the values of the first and second engine condition digital numbers; means for storing the resultant fuel digital word and for storing the resultant ignition digital word; means for generating one or more position signals when the engine is in one or more predetermined positions; first means responsive to one of the one or more position signals for producing said fuel signal at the end of a first predetermined operating interval; means for altering the value of the fuel signal as a function of the value of the resultant fuel digital word so that a proper amount of fuel is admitted to the engine through the fuel injection means; and second means responsive to one of the one or more position signals for producing the fire signal at the end of a second operating interval that is a function of the value of the resultant ignition digital word.
 21. Apparatus, as claimed in claim 20, wherein the first operating characteristic comprises engine speed and the second operating characteristic comprises the position of a member controlled by the operator of the engine.
 22. Apparatus, as claimed in claim 20, wherein the first and second means each comprises an analog to digital converter.
 23. Apparatus, as claimed in claim 20, wherein the data memory means comprises means for storing each fuel digital word and each ignition digital word at a predetermined address that represents a predetermined value of the first engine operating characteristic and a predetermined value of the second engine operating characteristic, and wherein the selecting means comprises: means responsive to the first and second engine condition digital numbers for selecting a plurality of the predetermined addresses, each selected address representing values of the first and second engine operating characteristics that at least approximately correspond to the values of the first and second engine operating characteristics represented by the first and second engine condition digital numbers; and means for deriving from the fuel and ignition digItal words stored at the selected addresses a single resultant fuel digital word and a single resultant ignition digital word.
 24. Apparatus, as claimed in claim 23, wherein the data memory means comprises means for uniquely identifying each predetermined address by a preselected digital first address number proportional to a preselected value of the frst operating characteristic and a preselected digital second address number proportional to a preselected value of the second operating characteristic.
 25. Apparatus, as claimed in claim 24, wherein the selecting means comprises: means for comparing the first address numbers with the first engine condition digital number and for comparing the second address numbers with the second engine condition digital number, and for selecting a plurality of selected addresses identified by first and second address numbers that approximate the values of the first and second engine condition digital numbers; and means for interpolating between the digital words stored at the selected addresses to produce the resultant fuel digital word and the resultant ignition digital word.
 26. Apparatus, as claimed in claim 20, wherein the selecting means comprises: an arithmetic unit for interpolating between the selected fuel and ignition digital words in accordance with the operating instructions.
 27. Apparatus, as claimed in claim 20, wherein the means for generating a position signal comprises: a first member mounted to a fixed portion of the engine; a second member mounted to a cylically moving portion of the engine so that the second member periodically passes adjacent the first member; and means for producing the position signal when the second member passes adjacent the first member.
 28. Apparatus, as claimed in claim 20, wherein the first means comprises: a clock generator for generating clock pulses at a predetermined rate; a counter; means for storing an open-time digital number having a value proportional to said first operating interval; means for loading the open-time digital number into the counter in response to the position signal; means for transmitting clock pulses to the counter so that the counter counts to a predetermined value; and means responsive to the counter for commencing the production of the fuel signal after the counter has counted to the predetermined value.
 29. Apparatus, as claimed in claim 20, wherein the means for altering comprises: a clock generator for generating clock pulses at a predetermined rate; a counter; means for loading the resultant fuel digital word into the counter in response to the commencement of the fuel signal; means for transmitting clock pulses to the counter so that the counter counts to a predetermined value; and means for terminating the fuel signal when the counter has counted to the predetermined value.
 30. Apparatus, as claimed in claim 20, wherein the second means comprises: a clock generator for generating clock pulses at a predetermined rate; a counter; means for loading the resultant ignition digital word into the counter in response to the position signal; means for transmitting the clock pulses to the counter so that the counter counts to a predetermined value; and means responsive to the counter for producing the fire signal after the counter has counted to the predetermined value.
 31. A method of injecting fuel into an internal combustion engine including a source of fuel for a fuel injection means that is opened to admit a predetermined quantity of fuel to the engine in response to an electrical fuel signal, said method comprising the steps of: storing a plurality of fuel digital words representing a plurality of discrete engine fuel requirements; storing a plurality of instruction digital words representing a plurality of operating instructions; initiating the operating instructions in a predetermined order; generatinG a first engine condition digital number having a value representing a first operating characteristic of the engine; selecting under control of the operating instructions one or more of the fuel digital words depending on the values of the first and second engine condition digital numbers so that each selected fuel digital word at least approximately represents the fuel requirement of the engine; deriving from the one or more selected fuel digital words a resultant fuel digital word representing the engine fuel requirement; storing the resultant fuel digital word; generating a position signal when the engine is in a predetermined position; producing said fuel signal at the end of a predetermined operating interval initiated by the generation of the position signal; and altering the value of the fuel signal as a function of the resultant fuel digital word so that the proper quantity of fuel is admitted to the engine through the fuel injection means.
 32. A method, as claimed in claim 31, wherein the step of storing comprises the step of storing each fuel digital word at a predetermined address that represents a predetermined value of the first engine operating characteristic and a predetermined value of the second engine operating characteristic, and wherein the step of selecting comprises the steps of: selecting a plurality of the predetermined addresses, each selected address representing values of the first and second engine operating characteristics that at least approximately correspond to the value of the first and second engine operating characteristics represented by the first and second engine condition digital numbers; and deriving from the fuel digital words stored at the selected addresses a single resultant fuel digital word.
 33. A method as claimed in claim 32, wherein the step of storing comprises the step of identifying each predetermined address by a preselected digital first address number proportional to a preselected value of the first operating characteristic and a preselected digital second address number proportional to a preselected value of the second operating characteristic.
 34. A method as claimed in claim 33, wherein the step of selecting comprises the steps of: comparing the first address numbers with the first engine condition digital number; comparing the second address numbers with the second engine condition digital number; selecting a plurality of the predetermined addresses identified by first and second address numbers that approximate the values of the first and second engine condition digital numbers; and interpolating between the fuel digital words stored at the selected addresses to produce the resultant fuel digital word.
 35. A method, as claimed in claim 31, wherein the internal combustion engine comprises a plurality of cylinders, wherein the fuel injection means comprises a separate fuel injector for each cylinder of the engine, wherein the fuel signal comprises a separate current pulse for independently operating each fuel injector, wherein the step of producing the fuel signal comprises the steps of producing separate current pulses for opening each fuel injector independently of the other fuel injectors, and wherein the step of altering comprises the step of individually altering the value of each current pulse so that a quantity of fuel proportional to the value of the resultant fuel digital word is separately admitted to the engine through each fuel injector.
 36. A method, as claimed in claim 35, wherein the step of storing comprises the step of storing a separate correction digital number for each cylinder of the engine, and wherein the step of altering comprises the steps of separately combining the value of the resultant fuel digital word with the value of each correction digital number so that a quantity of fuel is admitted to each cylinder in proportion to the combined values of the resultant fuel digital word and the correction digital number sTored for use with that cylinder.
 37. A method of controlling a member of an internal combustion engine that is controllable in response to an electrical control signal, said method comprising the steps of: storing a plurality of control digital words representing a plurality of engine control requirements; storing a plurality of instruction digital words representing a plurality of operating instructions; initiating the operating instructions in a predetermined order; generating a first engine condition digital number having a value representing a first operating characteristic of the engine; generating a second engine condition digital number having a value representing a second operating characteristic of the engine; selecting one or more of the control digital words depending on the values of the first and second engine condition digital numbers so that each selected control digital word at least approximately represents the control requirement of the engine; deriving from the one or more selected control digital words a resultant control digital word representing the engine control requirement; storing the resultant control digital word; generating a position signal when the engine is in a predetermined position; and controlling the control signal in response to the position signal as a function of the value of the resultant control digital word.
 38. A method, as claimed in claim 37, wherein the step of storing comprises the step of storing each control digital word at a predetermined address that represents a predetermined value of the first engine operating characteristic and a predetermined value of the second engine operating characteristic, and wherein the step of selecting comprises the steps of: selecting a plurality of the predetermined addresses, each selected address representing values of the first and second engine operating characteristics that at least approximately correspond to the values of the first and second engine operating characteristics represented by the first and second engine condition digital numbers; and deriving from the control digital words stored at the selected addresses a single resultant control digital word.
 39. A method, as claimed in claim 38, wherein the step of storing comprises the step of identifying each predetermined address by a preselected digital first address number proportional to a preselected value of the first operating characteristic and a preselected digital second address number proportional to a preselected value of the second operating characteristic.
 40. A method, as claimed in claim 39, wherein the step of selecting comprises the steps of: comparing the first address numbers with the first engine condition digital number; comparing the second address numbers with the second engine condition digital number; selecting a plurality of the predetermined addresses identified by first and second address numbers that approximate the values of the first and second engine condition digital numbers; and interpolating between the control digital words stored at the selected addresses to produce the resultant digital word.
 41. A method, as claimed in claim 37, wherein the controllable member is a means for producing a spark capable of exploding a fuel mixture inside the engine, wherein each control digital word represents an ignition time at which a spark must be provided, and wherein the resultant control digital word represents a spark timing requirement of the engine.
 42. In a control system for an internal combustion engine comprising first control means for controlling the operation of a first cylinder of the engine in response to a first control signal and second control means for controlling the operation of a second cylinder of the engine in response to a second control signal, improved apparatus for individually controlling the first and second control means comprising: means for generating a control diGital word having a value representing a control requirement of the engine at the predetermined operating condition; means for storing the control digital word; means for storing a first correction digital number having a value representing a control requirement characteristic of the first cylinder; means for storing a second correction digital number having a value representing a control requirement characteristic of the second cylinder; means for initiating the production of the first control signal and the second control signal; means for altering the duration of the first control signal so that the operation of the first control means is controlled in proportion to the combined values of the control digital word and the first correction digital number and for altering the duration of the second control signal so that the operation of the second control means is controlled in proportion to the combined values of the control digital word and the second correction digital number.
 43. Apparatus, as claimed in claim 42, wherein the means for storing the control digital word comprises a first register, wherein the means for storing the first correction digital number comprises a second register and wherein the means for storing the second correction digital number comprises a third register.
 44. Apparatus, as claimed in claim 42, wherein the means for altering comprises: means for adding the value of the first correction digital number to the value of the control digital word to produce a first corrected control digital word and for adding the value of the second correction digital number to the value of the control digital word to produce a second corrected control digital word; a clock generator for generating clock pulses at a predetermined rate; counting means; means for loading the first corrected control digital word into the counting means and for producing a first output signal when the counting means has been stepped by the clock pulses to a predetermined value; means responsive to the first output signal for terminating the first control signal; means for loading the second corrected control digital word into the counting means and for producing a second output signal when the counting means has been stepped by the clock pulses to a predetermined value; and means for terminating the second control signal in response to the second output signal.
 45. Apparatus, as claimed in claim 42, wherein the control system comprises a fuel system, wherein the first control means comprises a first fuel injection means for admitting fuel into the first cylinder, wherein the second control means comprises a second fuel injection means for admitting fuel into the second cylinder and wherein the control requirements each comprise a fuel requirement of a cylinder.
 46. A method of regulating a control function of an internal combustion engine so that the engine operates at a predetermined value of an engine operating characteristic, the method comprising the steps of: storing a target digital number representing the predetermined value of the engine operating characteristic; converting the value of the engine operating characteristic at a first point in time into a first test digital number; converting the value of the engine operating characteristic at a second point in time later than the first point in time into a second test digital number; producing a first logic signal if the second test digital number is greater in value than the target digital number; producing a second logic signal if the second test digital number is less in value than the target digital number; producing a third logic signal if the second test digital number is greater in value than the first test digital number; producing a fourth logic signal if the second test digital number is less in value than the first test digital number; storing a delta digital number representing a small change iN the value of the control function; changing the control function in an amount proportional to the value of the delta digital number in response to the first and fourth logic signals; changing the control function in an amount proportional to the value of the delta digital number in response to the second and third logic signals; changing the control function in an amount proportional to the inverse of the value of the delta digital number in response to the first and third logic signals; and changing the control function in an amount proportional to the inverse of the value of the delta digital number in response to the second and fourth logic signals.
 47. A method, as claimed in claim 41, wherein the control function is the quantity of fuel supplied to the engine, wherein the engine operating characteristic is engine speed, and wherein the delta digital number represents a small change in the quantity of fuel normally provided to the engine.
 48. A method for controlling the operation of an internal combustion engine including a source of fuel for a fuel injection means to admit a predetermined quantity of fuel to the engine in response to an electricl fuel signal and spark means for producing a spark capable of igniting the fuel inside the engine in response to a fire signal, said method comprising the steps of: generating a first engine condition digital number having a value representing a first operating characteristic of the engine; generating a second engine condition digital number having a value representing a second operating characteristic of the engine; storing a plurality of fuel digital words and a plurality of ignition digital words in pairs, each pair comprising a fuel digital word and an ignition digital word stored at a single predetermined address that represents a predetermined value of the first engine condition digital number and a predetermined value of the second engine digital number, said fuel digital words representing a plurality of engine fuel requirements and said ignition digital words representing a plurality of discrete spark requirements timing requirements of the engine; selecting one or more of the predetermined addresses depending on the values of the first and second condition digital numbers; deriving from the one or more fuel digital words and ignition digital words stored at the one or more selected addresses, a resultant fuel digital word and a resultant ignition digital word; storing the resultant fuel digital word; storing the resultant ignition digital word; generating a position signal when the engine is in a predetermined position; producing said fuel signal at the end of a predetermined operating interval initiated by the generation of the position signal; altering the value of the fuel signal as a function of the resultant fuel digital word so that the proper quantity of fuel is admitted to the engine through the fuel injection means; and generating the fire signal at the end of a variable operating interval initiated by the generation of the position signal, the operating interval being varied as a function of the value of the resultant ignition digital word.
 49. A method, as claimed in claim 48, wherein the first operating characteristic comprises engine speed and the second operating characteristic comprises the position of a member controlled by an operator of the engine.
 50. A method, as claimed in claim 48, wherein the step of storing comprises the step of identifying each predetermined address by a preselected digital first address number proportional to a preselected value of the first operating characteristic and a preselected digital second address number proportional to a preselected value of the second operating characteristic.
 51. A method, as claimed in claim 50, wherein the step of selecting comprises and steps of: comparing the first address numbers with the first engine condition digital number; comparing thE second address numbers with the second engine condition digital number; selecting a plurality of a predetermined addresses identified by first and second address numbers that approximate the value of the first and second engine condition digital numbers; and interpolating between the fuel and ignition digital words stored at the selected addresses to produce the resultant fuel digital word and the resultant ignition digital word. 